The use of laser lines as a tool for non-contact measurement gauging of an object is well established. Laser lines are projected onto the object being gauged, and an image of the projected lines is observed by one or more detectors for use in triangulation measurements. However, laser lines used in structured light systems for measurement gauging of objects contain coherent noise, such as speckle, which limits the performance of the system.
One approach to increase the measurement accuracy of laser based measurement gauges has been to reduce the size of the laser spot or laser line thickness projected onto the surface of the object undergoing measurement gauging. However, in the case of a smaller projected spot or thinner line projection, on the order of a few tens of microns in diameter or cross-section, the coherent speckle created by the surface characteristics of the object onto which the laser is projected can become very large, even compared to the size of the viewing lens, and hence are seen as continuous patterns of light at the detectors.
This problem has been previously addressed by scanning the laser spot or beam along the length of the projected line. This is often done by rapidly scanning a single laser point to form the projected image of a line, or merely moving a projected image of a laser line back and forth slightly in a dithering motion. Movement of the laser beam along the length of the projected line averages out the coherent speckle perceived as noise by the detectors. While these methods average out the coherent speckle noise, they reduce available light energy received by the detectors, and add additional sources of error associated with the position of the line due to the wobble present in the mechanical scanning or dithering systems.
Accordingly, there is a need for a method to reduce coherent speckle noise observed in projected laser lines used for the measurement gauging of an object, without reducing the light energy available for detection or introducing additional mechanical scan errors into the measurement system.